Role of rpoS in acid resistance and fecal shedding of Escherichia coli O157 : H7

Acid resistance (AR) is important to survival of Escherichia coli O157:H7 i n acidic foods and may play a role during passage through the bovine host. In this study, we examined the role in AR of the rpoS- encoded global stres s response regulator sigma(S) and its effect on shedding off. coli O157:H7 in mice and calves. When assayed for each of the three AR systems identifie d in E. coli, an rpoS mutant (rpoS::pRR10) of E. coli O157:H7 lacked the gl ucose-repressed system and possessed reduced levels of both the arginine- a nd glutamate-dependent AR systems. After administration of the rpoS mutant and the wild-type strain (ATCC 43895) to ICR mice at doses ranging from 10( 1) to 10(4) CFU, we found the wild-type strain in feces of mice given lower doses (10(2) versus 10(3) CFU) and at a greater frequency (80% versus 13%) than the mutant strain. The reduction in passage of the rpoS mutant was du e to decreased AR, as administration of the mutant in 0.05 M phosphate buff er facilitated passage and increased the frequency of recovery in feces fro m 27 to 67% at a dose of 10(4) CFU. Enumeration of E. coli O157:H7 in feces from calves inoculated with an equal mixture of the wild-type strain and t he rpoS mutant demonstrated shedding of the mutant to be 10- to 100-fold lo wer than wild-type numbers. This difference in shedding between the wild-ty pe strain and the rpoS mutant was statistically significant (P less than or equal to 0.05). Thus, sigma(S) appears to play a role in E. coil O157:H7 p assage in mice and shedding from calves, possibly by inducing expression of the glucose-repressed RpoS-dependent AR determinant and thus increasing re sistance to gastrointestinal stress. These findings may provide clues for f uture efforts aimed at reducing or eliminating this pathogen from cattle he rds.